This invention pertains to a method and apparatus for cooling and conditioning glass in preparation for forming processes. In particular there is disclosed a method and apparatus adapted for the manufacture of tubing. However, the invention described herein would be suitable for other glass forming processes as well.
A forehearth may be defined generally as a covered refractory channel in which glass is cooled and conditioned prior to forming. Such devices or structures have been in general use in the glass industry for many years. However, they leave much to be desired from the point of view of construction cost, energy consumption and operating performance.
The construction and repair costs of modern forehearths have increased to the point that such expenditures are a substantial portion of the total investment in the furnace. Thus, any reduction in this outlay would be a desirable advantage.
Forehearths are primarily intended to remove heat from glass, but the anomaly exists that, in order to properly condition the glass prior to forming, substantial amounds of energy must be selectively added. Accordingly, energy is wasted in order to regulate a waste heat operation.
Finally, since forehearths are relatively complex devices with long time lags, a great degree of art and skill is required to obtain optimum temperature uniformity at the exit end where forming takes place.
In a conventional forehearth, illustrated in FIG. 1, glass enters at the left and flows towards the orifice at the right. A typical cooling curve is illustrated in the same Figure showing temperature T vs. the distance D from the forehearth orifice. The surface glass temperature is significantly cooler than the bottom glass. Thus, heat is applied in order to condition the glass, with the objective of producing a more uniform glass temperature profile. There are many interactional effects, and thus, only partial uniformity is accomplished by trial and error.
The typical forehearth of the prior art has a relatively narrow width (W) in comparison to its length (D) and the velocity distribution of the glass varies from about twice the average velocity in the center of the channel on the surface (see FIG. 1 top view) to near zero at the sidewall. In order for the glass to remain at the uniform temperature, the heat loss must vary in proportion to the velocity. This is usually never accomplished naturally and the glass along the sidewall becomes cold. Side and surface glass may be heated with electrically energized submerged electric side heaters and overhead burners. This additional energy must be dissipated before the glass enters the orifice and thus the forehearth needs to be longer in order to handle the greater heat load. Thus, in order to condition the glass to a uniform temperature, heat energy input is required while at the same time heat is removed.
A number of systems are available in the prior art for conditioning glass prior to formation. One such invention of Lufkin, U.S. Pat. No. 2,038,797, illustrates a rotating reservoir fired by burners having an outlet portion wherein glass may be removed by hand or by other machinery. This system requires energy input to maintain the temperature of the glass within the rotating reservoir. Kadow, U.S. Pat. No. 1,815,258 shows a rotating cone and a reservoir portion upstream of forming means. This system requires one or more burners with which to maintain stable high temperature in the reservoir portion. Soubier, U.S. Pat. No. 1,967,378 is a system similar to Kadow and requires burners to maintain the heat.
A number of patents show systems wherein glass overflows a curb or forming ledge portion of a tube drawing apparatus. Some systems rotate while others do not. Typical examples are shown in other U.S. Pat. Nos. such as Favre, 1,899,891; Richardson, 1,933,341; Cardot, 1,949,037; Gray 2,133,662, and Weber, RE. 20,522. A significant problem with some of these devices, especially ones that have a rotating reservoir, is that the outlet orifice which is contiguous with the overflow curb receives an inordinate amount of wear, and it is difficult to control the wall thickness and minimize eccentricity of the tubing. Similarly U.S. Pat. Nos. to Howard, 1,823,543 and Soubier, 1,750,972 show systems where glass is formed by means of a mandrel and a rotating orifice, the former having a funnel-like outlet and the latter having a reservoir portion upstream of a tapered orifice. These systems suffer from the same problems of the previous group and also from the problem that it is difficult to maintain the mandrel forming device in the orifice concentrically with a rotating reservoir or funnel. Further, these systems do not approach the problem of conditioning the glass temperature to the extent of the present invention.
All the aforementioned systems, except for the Howard '543, require a pool or annular zone wherein glass is maintained in a reservoir-like structure. One of the problems with pool systems is that the bottom glass may become too cool to move, stagnant and devitrify. If the bottom glass does not move through the system, hot glass, which has not had the sufficient time to lose excess heat energy will short circuit flow to the orifice. Thus, glass and energy is wasted. In the somewhat different system of Howard, despite the claim that flow will be uniform, the glass will accumulate down stream near the orifice. Moreover, Howard is not designed for, nor does it address the problem of glass conditioning, since this is presumably accomplished in the forehearth section above the funnel.
Weber '522 discusses the conditioning of glass using a heat exchanger to control the temperature of the interior of a drawn tube, while leaving the glass near the overflow curb hot. The cooling device or heat exchanger near the outlet orifice helps to quickly harden the glass as the tubing is formed and has relatively little to do with the conditioning of the glass prior to formation.
It is important to note that the present invention allows for a better application of a tube drawing process known as the "VELLO" process. In a conventional draw, an overflow is provided to eliminate a cord known as a "VELLO streak" caused by asymetries in the temperature, flow and possibly composition of glass about the bell shaft. The overflow is not without its own problems, such as, producing a nonuniform head of glass above the orifice, thus, making it difficult to control tubing wall thickness and quality. As will be hereinafter described, the present invention provides a means whereby an improved tube draw may be utilized.
The present invention seeks to avoid some of the problems of the aforementioned prior arrangements and its object and functions are summarized below.